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(EX/S1-5) Beta-limiting MHD Instabilities in NSTX Spherical Torus Plasmas

J.E. Menard¹⁾, M.G. Bell¹⁾, R.E. Bell¹⁾, E.D. Fredrickson¹⁾, D.A. Gates¹⁾, S.M. Kaye¹⁾, B.P. LeBlanc¹⁾, R. Maingi²⁾, D. Mueller¹⁾, S.A. Sabbagh³⁾, D. Stutman⁴⁾, C.E. Bush²⁾, D.W. Johnson¹⁾, R. Kaita¹⁾, H.W. Kugel¹⁾, R.J. Maqueda⁵⁾, F. Paoletti³⁾, S.F. Paul¹⁾, M. Ono¹⁾, Y.-K.M. Peng²⁾, C.H. Skinner¹⁾, E.J. Synakowski¹⁾, NSTX Research Team
 
¹⁾ Princeton Plasma Physics Laboratory, Princeton, USA
²⁾ Oak Ridge National Laboratory, Oak Ridge, TN, USA
³⁾ Columbia University, New York, NY, USA
⁴⁾ Johns Hopkins University, Baltimore, MD, USA
⁵⁾ Los Alamos National Laboratory, Los Alamos, NM, USA

Abstract.  Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) beta-t = 35%, beta-N = 6.5, volume-average beta-N = 4.5, beta-N / li = 10, and beta-P = 1.4. High beta-P operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of beta-N = 6. Details of the beta limit scalings and beta-limiting instabilities in various operating regimes are described.

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